# std::size, std::ssize

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< cpp‎ | iterator

Defined in header `<array>` |  |
Defined in header `<deque>` |  |
Defined in header `<flat_map>` |  |
Defined in header `<flat_set>` |  |
Defined in header `<forward_list>` |  |
Defined in header `<inplace_vector>` |  |
Defined in header `<iterator>` |  |
Defined in header `<list>` |  |
Defined in header `<map>` |  |
Defined in header `<regex>` |  |
Defined in header `<set>` |  |
Defined in header `<span>` |  |
Defined in header `<string>` |  |
Defined in header `<string_view>` |  |
Defined in header `<unordered_map>` |  |
Defined in header `<unordered_set>` |  |
Defined in header `<vector>` |  |
template< class C > constexpr auto size( const C& c ) -> decltype(c.size()); | (1) | (since C++17)
template< class C > constexpr auto ssize( const C& c ) -> std::common_type_t<std::ptrdiff_t, std::make_signed_t<decltype(c.size())>>; | (2) | (since C++20)
template< class T, std::size_t N > constexpr std::size_t size( const T (&array)[N] ) noexcept; | (3) | (since C++17)
template< class T, std::ptrdiff_t N > constexpr std::ptrdiff_t ssize( const T (&array)[N] ) noexcept; | (4) | (since C++20)

Returns the size of the given range.

1,2) Returns c.size(), converted to the return type if necessary.
3,4) Returns N.

### Parameters

c | - | a container or view with a `size` member function
array | - | an array of arbitrary type

### Return value

1) c.size()
2) static_cast<std::common_type_t<std::ptrdiff_t,
std::make_signed_t<decltype(c.size())>>>(c.size())
3,4) N

### Exceptions

1,2) May throw implementation-defined exceptions.

### Overloads

Custom overloads of `size` may be provided for classes and enumerations that do not expose a suitable `size()` member function, yet can be detected.

Overloads of `size` found by argument-dependent lookup can be used to customize the behavior of std::ranges::size, std::ranges::ssize, and std::ranges::empty. | (since C++20)

### Possible implementation

size (1)
    template<class C>
    constexpr auto size(const C& c) -> decltype(c.size())
    {
        return c.size();
    }
ssize (2)
    template<class C>
    constexpr auto ssize(const C& c)
        -> std::common_type_t<std::ptrdiff_t,
                              std::make_signed_t<decltype(c.size())>>
    {
        using R = std::common_type_t<std::ptrdiff_t,
                                     std::make_signed_t<decltype(c.size())>>;
        return static_cast<R>(c.size());
    }
size (3)
    template<class T, std::size_t N>
    constexpr std::size_t size(const T (&array)[N]) noexcept
    {
        return N;
    }
ssize (4)
    template<class T, std::ptrdiff_t N>
    constexpr std::ptrdiff_t ssize(const T (&array)[N]) noexcept
    {
        return N;
    }

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_nonmember_container_access` | `201411L` | (C++17) | `std::size()`, std::data and std::empty
`__cpp_lib_ssize` | `201902L` | (C++20) | `std::ssize()` (2,4) and unsigned `std::span::size()`

### Example

Run this code

    #include <cassert>
    #include <cstring>
    #include <iostream>
    #include <vector>

    int main()
    {
        // Works with containers
        std::vector<int> v{3, 1, 4};
        assert(std::size(v) == 3);

        // And works with built-in arrays too
        int a[]{-5, 10, 15};
        // Returns the number of elements (not bytes) as opposed to sizeof
        assert(std::size(a) == 3);
        std::cout << "size of a[]: " << sizeof a << '\n'; // 12, if sizeof(int) == 4

        // Provides a safe way (compared to sizeof) of getting string buffer size
        const char str[] = "12345";
        // These are fine and give the correct result
        assert(std::size(str) == 6);
        assert(sizeof(str) == 6);

        // But use of sizeof here is a common source of bugs
        const char* str_decayed = "12345";
        // std::cout << std::size(str_decayed) << '\n'; // Usefully fails to compile
        std::cout << sizeof(str_decayed) << '\n'; // Prints the size of the pointer!

        // Since C++20 the signed size (std::ssize) is available
        auto i = std::ssize(v);
        for (--i; i != -1; --i)
            std::cout << v[i] << (i ? ' ' : '\n');
        assert(i == -1);

        // Note that the string literal includes the ending null character, which
        // will be part of the constructed characters array. This makes std::size
        // behave differently from std::strlen and std::string::size:
        constexpr char symbols[] = "0123456789";

        static_assert(std::size(symbols) == 11);
        static_assert(std::string(symbols).size() == 10);
        assert(std::strlen(symbols) == 10);
    }

Possible output:

    size of a[]: 12
    8
    4 1 3

### See also

ptrdiff_t | signed integer type returned when subtracting two pointers (typedef)
size_t | unsigned integer type returned by the `sizeof` operator (typedef)
ranges::size (C++20) | returns an integer equal to the size of a range (customization point object)
ranges::ssize (C++20) | returns a signed integer equal to the size of a range (customization point object)

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